/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineSessionInterface.h
Sends an invitation to play in the player's current session
true if successful, false otherwise
bool SendSessionInviteToFriend ( int32 LocalUserNum, [FName](API\Runtime\Core\UObject\FName) SessionName, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & Friend )